Photoconductive device and methods of making same



United States Patent This invention relates to improvements in methods for production and treatment of photosensitive materials and more specifically to new and improved methods for producing large mono-crystals of cadmium sulfide and like photosensitive compounds and for treating crystals and coalescent layers of such compounds to obtain predetermined desired response characteristics particularly to X- ray radiation, and to articles thus produced.

It has heretofore been disclosed that crystals of cadmium sulfide and like compounds may be grown by controlled condensation from the vapor phase, a full disclosure of one such method being given by Frerichs in the October 1, 1947, issue of Physical Review, volume 72, No. 7, at pages 594-601. In one important aspect, the presentinvention provides basic improvements on these prior vapor phase crystal growth processes, the crystal growth processes of the invention being particularly advantageous in requiring less precise control of temperature during the growing operation and in affording better control of crystal size and thickness than prior processes such as that of Frerichs.

It is known that cadmium sulfide and similar com? pounds, including the sulfides, selenides, tellurides and oxides of the metals zinc, cadmium and mercury, are photosensitive to X-ray radiation, their conductivity varying with intensity of the incident radiation. Such materials, preferably in the form of large single crystals or nongranulate coalescent layers, therefore may be used for the detection and measurement of X-ray radiation. For maximum ease and extent of application it is highly desirable that the X-ray sensitive crystal or layer have X-ray response characteristics substantially similar to those of the conventional ionization chamber type X-ray sensing unit, so as to enable direct substitution of the crystal or layer into the measuring circuits commonly used with these ionization chamber instruments. This would permit use of CdS and like photoelements interchangeably with ionization chamber elements in a single measuring circuit with little or no modification thereof, but such interchangeability has heretofore been impossible because of the basically different response characteristics of ionization chamber and photoconductive type sensitive elements.

ionization chamber X-ray units normally present an effective resistance which is inversely proportional to the intensity of the incident radiation, i.e., its response is substantially linear with varying intensity of radiation of constant wavelength. Ionization current also varies with radiation wavelength and, accordingly, with X-ray voltage.

Cadmium sulfide and like photocrystals as heretofore produced do not possess response characteristics matching the ionization chamber as is necessary to permit their use interchangeably therewith. iln general, photoconductivity in such crystals does not increase proportionally to the intensity of incident radiation, but rather increases more slowly and in the limiting condition increases proportionally with the square root of the absorbed energy. While some few crystals produced by prior methods do have this required linearity of response, such crystals almost invariably are relatively insensitive and do not provide readily measured resistance changes. Moreover, in few if any such prior crystals does the photoconductance 3,104,365 Patented Sept. 17, 1963 vary with radiation voltage and wavelength in the same or similar manner to the air ionization elements.

In accordance with the invention, crystals of cadmium sulfide and other compounds as listed above may be produced and their response characteristics modified to provide the desired correspondence with ionization cham her response, so as to afford full interchangeability therewith and accurate X-ray dosage measurements in units. Crystal treatment according to the invention permits systematic linearizing of response of high sensitivity crystals, with any unavoidable loss in sensitivity incident thereto being very small and no greater than absolutely necessary.

For X-ray measurement, a cadmium sulfide or like crystal grown and its response characteristics modified by the methods of the invention has the following advantages over the conventional ionization chamber unit: (1) The voltage applied to the crystal may be very small particularly if maximum sensitivity is not required, a few volts as provided by a tube plate or heater battery usually is adequate; (2) The photoconductance currents normally are several times higher than ionization currents as usually obtained, hence only common instruments are required for current measurement; and (3) The small size of the crystal makes its use as a radiation dosimeter possible where the size of the conventional ionization chamber is an obstacle, thus permitting use of the crystal type dosimeter within the human body and also in making spot measurements of X-ray intensity.

Accordingly, it is a primary object of the invention to provide new and improved methods for producing and modifying the response characteristics of radiant energy sensitive materials including the sulfides, selenides, tellurides and oxides of the metals zinc, cadmium and mercury, and to novel articles as produced by these methods.

It is also an important object of the invention to provide novel methods for producing large single crystals of cadmium sulfide and like radiant energy sensitive materials.

Another object of the invention is the provision of novel methods for modifying the response characteristics of cadmium sulfide and like crystals so as to match those of the conventional ionization chamber units for interchangeability of use therewith particularly in X-ray measurement.

It is a further object of the invention to provide new and improved methods for systematically linearizing the response of cadmium sulfide and like crystals to X-ray irradiation and to obtain predetermined desired correspondence of response with varying X-ray voltage and wavelength.

These and other objects, features and advantages of the invention will become more fully apparent by reference to the appended claims and the following detailed description.

The Frerichs article cited above discloses a method for growing single crystals of cadmium sulfide in steps of placing a quantity of cadmium metal in a small porcelain boat and heating the same within an elongated closed growing tube, which may be of quartz, to a temperature of about 800 to 1000 C. Aslow stream of hydrogen gas is passed through the tube and over the boat therein by a hydrogen supply conduit opening into one end of the growing tube, suitable exhaust means being provided at the other end. Cadmium vapor rising from the boat is entrained in the hydrogen flow and moves with it to a point adjacent the downstream end of the boat, where it mingles with hydrogen sulfide introduced there by a second supply conduit means. A reaction then occurs by which cadmium sulfide vapor is condensed in crystalline form within the growing tube. By slowly cooling down, with the hydrogen and hydrogen sulfide gases still flowing, crystals of very pure photoconductive cadmium sultide are said to be obtained, with good uniformity of re- 'just described.-

' temperature. I

produced with the desired linearityof response in the spouse characteristics from crystal to crystal. and from batch to batch. r

In accordance with the invention, thisand like crystal quires that slightly higher temperatures, be reached during the growing process, but the procedural steps followed and apparatus .used maybe otherwise identical to those a In the crystal growing procedures of the invention the growing temperature, while higher, is much less critical to satisfiactory crystal growth,;thus significantly improving the crystal yield- This greater permissible variation in temperature affords better control of the size and thickness of the crystals grown.

. in-g material in the prior processes. This substitution revery high photosen-sitivity to visible radiation and accordingly are well adapted to use as photoelements' of general utility. They also are highly photoconductive to X-ray.

irradiation, though as initially produced they do not possess the desired correlation with ionization chamber response characteristics as explained in the foregoing.

The necessary change in crystal response characteristics to obtain this desiredcorrelation may in accordance with one method of the invention be obtained by exposing the crystals to high energy corpuscular radiation, as for example alpha particle bombardment. The extent of the change in response oharacteristicsdepends on the number of particles absorbed by the crystal, :andit therefiore may be precisely controlled by careful selection of the intensity and duration of irradiation. For example, to produce cadmium sulfide crystals having the desired linearity of response to X-rays in medium voltage range and of common intensities, the crystals should be irradiated with approximately 10 alphaparticles per sq. cm. A second method effective to linearize crystal response is to leave the crystal for an extended period of time exposed to relatively high temperature. The effects on the crystal may be controlled by varying the length of time heat treatment is continued and also by varying the Highly sensitive cadmium sulfide may be X-ray range after a short annealing at about 500 C., followed by slow cooling.

The above-described methods for linearizing crystal response characteristics relate not only to theiruse in X-ray measurement, but also are useful generally for 7 radiation of other types and particularly quantum radiation and corpuscular radiation.

- The present invention further comprehends methods I for compensating the X-ray Wavelength dependency of cient. This dilierence causes the crystal photocurrent change to be not directly proportional, to the value as the X-ray voltage and wavelength are varied. According to the invention, these deviations may be substantially corrected'by placing over the crystal, in the path of the X-ray beam, an. absorbing layer or filter whose thickness and material is dependent on the nature of the deviation of photocurrent fromv the 7 value. Preferably, a filter material whose absorption spectrum for X-rays is exactly the same as or similar to the absorption spectrum of the crystal should be used. For example, if the crystal is of cadmium sulfide the filter should be either of cadmium, tin or silver. j

The crystal electrodes may in accordance with the invention be utilized as such a compensatory filter by proper selection of electrode material and thickness on at least the cathode side of the crystal.

The correction or compensation of wavelength dependency is easier if crystals absorb only a small portion of the incident energy. It therefore is desirable to make the crystals as thin as possible, and in practice relatively thin large area crystals' are preferred.

The'invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are thereforeto be considered in all respects as illustrative and not restric tive, the scope of the invention being indicated by the appended claims rather than by the foregoing'description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to bet embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1; The method of growing large single crystals of cadmium sulfide and linearizing response characteristics: thereof comprising the steps of heating powdered cadmium sulfide to produce a vapor thereof, introducing hydrogen and hydrogen sulfide gases to serve as a reducing agent and as a vehicle for carrying the vapor to a region of lower temperature thereby causing the vapor to condense and form as a sublimate large single crystals of cadmium sulfide, and, then exposing said single crystals to highenergy particle irradiation for linearizing the response characteristics of said crystals.

2. In the method of growing cadmium sulfide monocrystals which include the steps of vaporizing cadmium sulfide in a hydrogen atmosphere and introducing into the vapor thus produced hydrogen sulfide, transferring the vapor mixed with the hydrogen sulfide along a confined path to a region of lower temperature whereby cadmium sulfide condenses from the vapor phase in monocrystalselected from the group consisting of -the sulfides of Zinc,

cadmium and mercury, which includes the step of su jecting the crystals to radiation by alpha particles.

4. A photosensitive element particularly for X-ray dosimeter use comprising a single crystal of a radiant energy sensitive compound selected from the group consisting of the sulfides of zinc, cadmium and mercury, having on at least one side thereof an electrode of a material having X-ray absorption characteristics similar to the absorption characteristics of'the crystal such that crystal photoconducance responsive to X-ray irradiation is correlated to wavelength of the incident radiation in predetermined desired manner. i

5. The photoelement defined in claim 4 wherein the radiant energy sensitive compound is cadmium sulfide and the electrode material is selected from the group consisting of cadmium, tin and silver.

' 6. The method defined in claim 2 including the further step of annealing the crystals grown by heating the crystals to a 'temperaure of the order'of 500 C., followed by slow cooling to linearize the crystal response.

7. The method defined in claim Zincluding the further step of subjecting said grown crystal to radiation by alpha particles.

8. The method defined in claim 7 including the-step of depositing on at least one side of said grown crystal an electrode of a material having X-ray absorption characteristics similar to the absorption characteristics of said crystal. V

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES M61101: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 4, 1923, pp. 589 and 6 02. 1923 Frerichs: Physical Review, volume 72, No. 7, 1947, 1 1. Thomson Apr. 3, 1923 5 594 601 May 1923 Hofstadter: Physical Review, volume 72, No. 7, 1947, Ca-Skhll May 13, 1930 pp. 1120H2L Andres 1936 Lappand Andrews: Nuclear Radiation Physics, 1948, Betterton et a1 Mar. 31, 1936 3Q2 3 Shockley Jan- 19 1954 0 Glasstone: Principles of Nuclear Reactor Engineering Ravich June 16, 1959 (1st edition 1955), pp. 66 and 67. 

1. THE METHOD OF GROWING LARGE SINGLE CRYSTALS OF CADMINUM SULFIDE AND LINEARIZING RESPONSE CHARACTERISTICS THEREOF COMPRISING THE STEPS OF HEATING POWDERED CADMIUM SULFIDE TO PRODUCE A VAPOR THEREOF, INTRODUCING HYDROGEN AND HYDROGE SULFIDE GASES TO SERVE AS A REDUCING AGENT AND AS A VEHICLE FOR CARRYING THE VAPOR TO A REGION OF LOWER TEMPERATURE THEREBY CAUSING THE VAPOR TO CONDENSE AND FORM AS A SUBLIMATE LARGE SINGLE CRYSTALS OF CADMIUM SULFIDE, AND THEN EXPOSING SAID SINGLE CRYSTALS TO HIGH ENERGY PARTICLE IRRADIATION FOR LINEARIZING THE RESPONSE CHARACTERISTICS OF SAID CRYSTALS. 